This invention relates to improvements in electric power assisted steering systems.
In an electric power assisted steering system, an electric motor is used to provide an assistance torque for the driver, reducing the effort required to steer the vehicle. The motor is connected to the steering system and applies a torque that is dependent on the torque applied to the handwheel of the steering system by the driver. This is typically measured using a torque sensor connected to the steering shaft which supports the handwheel. A motor controller receives the output signal from the torque sensor and from this produces a motor torque demand signal that is passed to a motor drive circuit which produces the motor currents needed to drive the motor.
To provide for enhanced levels of integrity in the system, it is also known to provide two output signals from the torque sensor, or even to provide two torque sensors which each provide a torque measurement. The two output signals are cross-checked to give a degree of confidence that the torque value indicated by the signals is correct. If there is a difference between the torques indicated by each of the two signals, then this indicates that the sensor signals cannot be trusted. In the prior art this has meant that the EPAS motor has to be disabled, leaving the driver with no assistance. At low speeds such as those used for parking (less than 5 miles per hour), it is very difficult to steer a modern vehicle without the power assistance provided by the motor. The vehicle is effectively disabled until the fault can be diagnosed and fixed by a service centre.
In many cases, only one output signal or “channel” of the sensor fails, so that one of the two output signals remains valid. In these cases, it would be desirable to continue to provide the steering assistance function (possibly with some degradation in steering performance) using the valid output signal. This would allow the driver to “limp home”.
The applicant has appreciated that it is not always possible to identify with any confidence which channel of the sensor has failed and which one is correct. Moreover, if the remaining valid signal is used to control the assistance and it subsequently fails, then it may potentially give an incorrect signal to the EPAS controller, leading to an incorrect assistance torque. This is undesirable and is hard to detect as there is no longer an additional channel against which to perform a check. Therefore, the applicant has appreciated that it would be desirable to provide a further means of monitoring the torque sensor operation when there is a significant discrepancy between the torque indicated by the signals from each channel to determine which of the two channels (where provided) should be used in the limp-home mode, and to check that the channel being used in a “single-channel” operating mode is operating correctly.